The present invention relates generally to carburetors for internal combustion engines, and more specifically to internally vented float bowl carburetors.
In a typical float bowl type carburetor, fuel flows from the reservoir in the float bowl through a fuel metering orifice into a fuel well from which the fuel is drawn up and mixed with air due to the pressure differential caused by the venturi region in the carburetor bore or throat. Since a continuous flow of fuel from the float bowl to the venturi must be provided in order to assure smooth engine operation, it is necessary to maintain a consistent fuel level in the float bowl. In order to obtain a consistent fuel level, a float control valve arrangement is provided such that as the fuel level in the float bowl is depleted through normal operation, the float control valve opens a conduit connected to a larger fuel storage tank. As fuel is replaced in the float bowl, an excess pressure is created above the fuel level. A proper fuel flow rate is facilitated by venting this excess pressure from the top of the float bowl to a constant pressure region. This venting may be to the atmosphere external of the carburetor (external venting) or to a region of relatively constant pressure close to atmospheric pressure within the carburetor bore (internal venting). Both types of venting arrangements are well known in the art.
Internally vented float bowl arrangements are advantageous to externally vented bowls in that air that is supplied to the vent has already passed through the carburetor air filter so that the likelihood of introducing additional contaminants into the carburetor is greatly reduced. In addition, in internally vented arrangements, as the air cleaner element becomes clogged and the pressure within the carburetor throat decreases, the pressure in the fuel bowl also decreases due to the passageway connecting the carburetor throat and bowl, thereby leaning out the fuel/air mixture to a level proportional to the level that existed when the air cleaner element was unobstructed.
In internally vented bowl arrangements, the venting function is influenced by conditions downstream of the air cleaner. For example, the internal bowl vent is subject to a relatively strong varied signal wave or pressure pulse that is produced upon the intake stroke of the engine. To a lesser extent, the pulse signal to the internal vent is affected by other engine factors such as valve and piston ring seals. In addition, pulse signal fluctuations to the internal vent are amplified in larger displacement engines, i.e. twenty cubic inches and larger, due to increased fuel demands. Pulse signal fluctuation to the internal vent is undesirable in that it results in an inconsistent flow of fuel from the float bowl to the venturi. As the vacuum in the fuel bowl increases, vent signal fluctuation is magnified.
It is desired to provide an internally vented carburetor arrangement that can be incorporated into current small engine designs, yet provides improved calibration consistency as well as maintains air cleaner blockage compensation.